1. Field of the Invention
The present invention relates to a data communication control system for a network system composed of plural communication stations connected to a communication medium, and more particularly to such system with a token passing method in which the right of communication is shared in time by circulating the destination address of the token in the order of logic addresses.
2. Description of the Prior Art
In a data transmission system with a conventional token passing method, efficient and well-balanced communication without collision of data on the transmission channel is achieved by circulating a token, representing a command for transferring the right of communication, through all the stations connected to the network, and giving the right of communication to a station which has received the token.
In such a conventional method, however, the token is always circulated in a predetermined order though the stations connected to the transmission channel may have respectively different frequencies of requesting the use of the transmission channel.
For example, a work station, provided with a keyboard, a display unit etc. for processing data, such as image information, by an operator, has a relatively limited frequency of requesting the use of the transmission channel because of the nature of function of such station, but a station commonly utilized by plural stations connected to the transmission channel, such as a file server used as a memory unit for various data, has a higher frequency of access, thus involving a much higher frequency of requesting the use of the transmission channel. Still the token is circulated to all the stations equally, so that the requests for the use of the transmission channel cannot be satisfactorily fulfilled. For this reason, in order to fully exploit the performance of such a commonly utilized station, it has been proposed to increase the signal transfer rate itself, but such an increase of the transfer rate has a limit and inevitably gives rise to a significant increase in the cost of all the stations connected to the transmission channel.